When I was learning DC-DC converters, I think they are a "DC version" of transformer (which is for AC-AC). So when I was designing a 100-30V flyback converter, my suggestion is: if a user inputs a DC 100V source, he gets a 30V output; if he inputs a DC 50V source, he gets a 15V. Just like how transformers act with AC, which have a fixed output/input ratio.
And someone then told me: no. If he inputs a 100V he gets 30V output; if he inputs a 50V, he should still get a 30V; even if he inputs a 15V, he still have to get a 30V. (We were talking about flyback converter. I'm not sure what about buck/boost converter. ) This is like a DC power supply (or it is), with which you want a fixed output.
So I'm curious and a little confused now about the actual situation. Do manufacturers always make DC-DC converters with fixed output voltage? Or with fixed output/input ratio? Or both are possible? In this case, is one of them are more often to use then the other?
The "someone" was like "How dumb are you! How can you even don't know this! " while telling me the story. I think it's more possible what he said makes more sense, because I'm really stupid. But what confused me is: if the actual situation is we never design DC-DC converters, like buck or boost or flyback or any other DC-DC converters, with a fixed output/input ratio, then how do I do if I want a "DC transformer" which give me output = f(input) = input * 3? Or, I don't know, maybe we will never want a DC transformer like this?